1. Field of the Invention
This invention relates to a turret structure for vessels such as offshore drilling or production vessels, and more particularly to a turret bearing arrangement that can be replaced in situ.
2. Background Art
Vessel mooring systems are known in the art in which a vessel may weathervane about a turret that is moored to the sea floor. The turret extends through a moon pool or cylindrical opening that extends through the hull of the vessel. The vessel is supported for rotation with respect to the turret by bearing structures placed between the turret and the vessel. Horizontal and vertical bearings transfer horizontal and vertical loads between the turret and the vessel.
FIG. 1 discloses such a turret mooring system, which is more fully described in co-pending U.S. patent application Ser. No. 13/446,857 filed on Apr. 13, 2012 in the names of Lindblade et al., which is incorporated herein by reference. FIG. 1 shows the bow of a vessel 10 having a well or moon pool 12 extending through the hull of vessel 10. Mounted within well or moon pool 12 is a turret generally indicated at 14 about which vessel 10 may weathervane. Well 12 is normally of a circular cross section, and turret 14 is typically of a cylindrical shape to fit within well 12. Anchor legs 16 are connected to turret 14 and may be secured to the sea bed by suitable anchors to restrict rotation of turret 14. Risers 18 extend to subsea wellheads or distribution facilities on the sea floor and are connected to turret 14. A manifold deck 20 is supported on the upper end of turret 14 and includes valves connected to risers 18. A swivel stack shown generally at 22 extends upwardly from turret 14 and manifold deck 20 and allows fluids to be transferred from the turret 14 to the vessel 10. Turret 14 is rotatively supported on vessel 10 by an upper bearing assembly 2 and an optional lower bearing assembly 15.
FIG. 2 discloses a turret upper bearing assembly of the rail and wheel variety, which is known in the art. Downward axial loads are transferred from the turret to the vessel using one or more rows of wheels 62 that run between rails 60 mounted on the vessel and rails 42 mounted on the turret. Wheels 62 includes inner and outer flanges that lip over the upper and lower rails to keep the wheels coaxially engaged with the rails. Radial loads are transferred from the turret to the vessel using circular rail 38 which is in contact with a circumferential arrangement of wheels 35 mounted on spring packs 74. Any uplift loads are transferred from the turret to the vessel using flanged wheels 70 that run between rails 40 mounted on the turret and rails 66 mounted on the vessel. In some applications, bogey wheel assemblies may be used in place of conventional wheels for the axial bearings, such as in an arrangement disclosed in U.S. Pat. No. 6,269,762 issued on Aug. 7, 2001 to Commandeur, which is incorporated herein by reference. In bogey wheel arrangements, the faces of the wheels only bear against a single rail; the wheel axles are mounted in pivoting carriers and form part of the path of components through which force is transferred.
Wheel and rail bearing assemblies, such as shown in FIG. 2 or in the Commandeur patent, are bulky and somewhat restricted in their load capacity. In addition, wheel and rail bearings are typically not sealed but are instead exposed to the harsh elements of the sea environment. This characteristic means that these bearings are subject to corrosion and failure and therefore require regular maintenance. The obverse side of this coin and one of the main advantages of wheel and rail bearings, however, is that wheels or rail sections can be individually replaced in-situ.
An alternative to the wheel and rail type bearing assembly that is known in the art is an integrated three-row roller bearing assembly. An example of such a bearing assembly is shown in FIG. 3 and is described in U.S. Pat. No. 5,893,784 issued in Apr. 13, 1999 to Boatman, which is incorporated herein by reference. The three-row roller bearing assembly 80 includes a circular array of rollers 82 radially aligned about turret 14 for supporting the turret weight. Any uplifting turret force is resisted by a second set of radially aligned rollers 84. A third set of rollers 86, coaxially aligned about turret 14, serve to transfer radial loads between vessel 10 and turret 14. Only one of each roller 82, 84, and 86 is visible in FIG. 3. Rollers 82, 84, 86 differ from the wheels of wheel and rail bearings in that they are flangeless and are typically characterized by an axial length equal to or greater than the diameter. All three bearing rows 82, 84, 86 of the three-row roller bearing assembly 80 are preferably lubricated and sealed within a common volume by seals 88, 89 to provide protection from the elements and prevent corrosion. The large number of rollers also result in a load bearing capacity that is superior to that of a wheel and rail bearing of the same diameter.
The integrated three-row roller bearing is a precise assembly that requires a high degree of flatness for proper load distribution and is somewhat intolerant of distortions and deflections, which cause high point loading stresses on select rollers. Accordingly, the integrated three-row roller bearing is typically bolted to the vessel via a spring mounting system (see FIG. 1 of the Boatman patent, for example) so as to isolate the bearing from vessel deformations due to wind, waves, currents, and loading.
A primary disadvantage of three-row roller bearings is that large roller bearings are manufactured offsite and are installed as complete units in the shipyard when the turret assembly and moon pool are constructed. As such, roller bearing components have heretofore not been replaceable in situ as have been wheel and rail bearing components. Although this has not typically been a problem, because the on-station service life of FPSO vessels is extending into multiple decades, owners of such systems are demanding in-situ replacement of the bearings.
3. Identification of Objects of the Invention
A principal object of the invention is to provide a turret bearing for a mooring turret system that combines the advantages of higher load capacity, increased reliability, and sealed environment characteristic of the integrated three-row roller bearing with the advantage of in situ replacement characteristic of wheel and rail bearings.
That is, an object of the invention is to provide a turret bearing that allows in situ replacement of its components;
Another object of the invention is to provide a turret bearing with a high load capacity; and
Another object of the invention is to provide a turret bearing with components that are sealed, enclosed and lubricated.